1. Field of the Invention
This invention relates to a method for directly measuring the amount of surface cold work due to a surface treatment operation. Such methods of this type, generally, allow the surface cold work created by a shot peening operation to be directly measured without having to destruct the substrate.
2. Description of the Related Art
Plastic strain is the mechanism whereby the surface is put into a state of compressive residual stress and it can further increase dislocation density, these being the key life enhancing benefits of the peening process. However, if the total cold work during peening exceeds the ductility limit of the material surface, embrittlement may result which can enhance crack initiation and growth. Excessive cold work is, therefore, an important damage mechanism for the process. Means to nondestructively measure the amount of cold work due to peening is critical for guaranteeing the life of high performance mechanical components.
Cold work is the result of the two shot peening variables, intensity and coverage. Intensity, or the energy imparted to the surface by individual impacts effects the amount of plastic strain induced in the surface by each shot strike. Coverage, on the other hand, is a measure of the number of times each element of the surface is cold worked during a shot peening operation. A coverage of 200% indicates that on average each point of the surface has been struck twice during the peening operation.
Methods to measure intensity from profilometer traces of a shot peened surface are disclosed in U.S. Pat. No. 5,003,805, entitled "Method and System for Monitoring Shot Peening" to R. A. Thompson and assigned to the same assignee as the present invention and U.S. patent application Ser. Nos. 646,957 and 650,828 entitled "Non-Destructive Monitoring of Surfaces by 3-D Profilometry" and "Quality Assurance of Surface Treatments by Analysis of Surface Line Traces", respectively, both to Thompson et al. and assigned to the same assignee as the present invention. Other methods to measure intensity would also apply. Similarly, U.S. patent application Ser. Nos. 764,034 and 850,732, respectively, entitled "Measurement of the Shot Peening Coverage by Automated Analysis of Peened Surface Line Traces" and "Measurement of Shot Peening Coverage by Impact Dent Characterization", both to Thompson et al. and assigned to the same assignee as the present invention, describe ways to use surface profile data to measure coverage, or the number of strikes the surface experiences during a peening operation.
While these methods have met with a modicum of success, none of these systems employ both intensity and coverage information to determine cold work, and the extent to which a peened surface is approaching embrittlement and the risk of fatigue or stress corrosion cracking failure. Therefore, a more advantageous system, then, would be presented if such amounts of embrittlement and fatigue could be determined.
Equally important to the shot peening of complex surfaces is the fact that the number and intensity of strikes may vary with part shape at different locations as the sprayed shot stream passes over them. At locations where excessive strikes occur, excessive cold work may lead to embrittlement and in increased risk in failure. This effect may be most severe, in fact, in critical, high stress regions such as fillets and corners, where ricochets can greatly increase the number of bombardments and impacts on sharp corners can greatly increase local intensity.
For this reason, conventional methods of determining intensity and coverage by Almen strips and die tracers may fall short. Similarly, NDE methods of measuring cold work take on added importance. Even advanced computational methods for calculating cold work from intensity and coverage models of complex geometrical regions require NDE verification before they can be used with confidence. Nevertheless, further reductions in NDE vertification would be advantageous.
It is apparent from the above that there exists a need in the art for a system that measures surface cold work due to a shot peening operation, and which measures the surface cold work in a non-destructive manner, but which at the same time is capable of determining the extent to which a peened surface is approaching embrittlement and the risk of fatigue or stress corrosion cracking failure. It is a purpose of this invention to fulfill this and other needs in the art in a manner more apparent to the skilled artisan once given the following disclosure.